1. Field of the Invention
This invention relates generally to a defocused unicell photometer with aspheric surface and, more particularly to a defocused unicell photometer with aspheric surface for providing select directional response characteristics without increasing the extreme field of view.
2. Background of the Prior Art
Use of unicell photometers for evaluating scene light to predict and/or control photographic exposures is well known. Such photometers generally consist of a photoresponsive transducer and a lens element for receiving actinic radiation from a scene and directing it onto a photosensitive surface of the transducer. The transducer may be either of the photovoltaic or photoresistive type and provide an output signal indicative of the actinic radiation incident to its photosensitive surface. The output signal from the transducer may thereafter be utilized to predict a photographic exposure condition or to actually control the closing of an automatic shutter blade mechanism at the appropriate film exposure condition. The output signal from such photometers is directionally responsive to the influence of actinic radiation from various scene objects within its field of view as based on their angular field position. Thus, brightly reflective scene objects which are located substantially on-axis with respect to the optical axis of the photometer contribute to a significantly greater portion of the output signal from the photometer than identical objects substantially displaced off the optical axis of the photometer. Since the directional response characteristic of most photometers is inherently so heavily weighted in favor of brightly reflective scene objects located substantially on-axis with respect to the optical axis of the photometer, it becomes generally desirable to suppress the on-axis response of the photometer while simultaneously increasing its off-axis response to provide a directional response substantially less sensitive to the position of a brightly reflective photographic subject in the field of view of the photometer. However, there is no absolute best directional response curve that would have universal photographic application since light distribution patterns will vary greatly between various photographic scenes. Thus, it becomes desirable to provide a means by which select directional response patterns may be achieved in a simple and economical manner without increasing the number of components customarily utilized in a unicell photometer.
One photometer, described in U.S. Pat. No. 3,162,766 entitled "Photoelectric Receiver with Supplementary Iris Diaphragm" by Ploke et al., issued Dec. 22, 1964 discloses a photoelectric receiver having a light diffusion body arranged between a photoelectric receiver and iris diaphragm. The light diffusion body is in the form of a lens wherein in one of the two faces of the light diffusion body there is included an aspheric for the purpose of obtaining a stronger concentration of the incoming parallel beam of light. The photoelectric receiver of Ploke, however, falls within the category of photometers known as "focused" thereby limiting the change in the directional response that may be achieved through the use of an aspheric.
Therefore, it is a primary object of this invention to provide an improved defocused unicell photometer wherein the directional response of the photometer may be specifically tailored to suit almost any directional response requirement imposed thereon.
It is another object of this invention to provide an improved defocused unicell photometer wherein the directional response curve for the photometer is suppressed with respect to on-axis actinic radiation incident thereto while being simultaneously broadened with respect to incident off-axis actinic radiation incident thereto without any change in the extreme field of view of the photometer.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the mechanism and system possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.